I am starting on a more adventuresome project, a 300B amp based on the classic JC Morrison 6SN7 design, using a regulated DC power for the heaters.
I understand this is sacred ground I am stepping into, and this irreverent amp I am building, using cheap Edcor iron, I'm sure will draw the ire of some, but I'm doing this for the shear fun of education and the Zen peacefulness of the construction process.
I'm robbing the iron out of a Blueglow/Kegger KT88 build I built, that I'm no longer using. The main expenses at this point is the chassis ($40), some various passives and some 300B-Z tubes with sockets ($225 on sale). I will probably end up in another $100 in misc parts if I don't melt anything in the process.
You can follow along with this build on via this YouTube playlist. Right now I've posted a couple of thought process videos in the planning stages. I've collected most of the parts, and the chassis should be here next week to dive into the metal work.
I have a feeling this one will end up in a different place than my initial planned path, but I do want to stick with a coupled 6SN7 driving a 300B as the end point.
300B DIY Budget Tube Amplifier Build Series - YouTube
I understand this is sacred ground I am stepping into, and this irreverent amp I am building, using cheap Edcor iron, I'm sure will draw the ire of some, but I'm doing this for the shear fun of education and the Zen peacefulness of the construction process.
I'm robbing the iron out of a Blueglow/Kegger KT88 build I built, that I'm no longer using. The main expenses at this point is the chassis ($40), some various passives and some 300B-Z tubes with sockets ($225 on sale). I will probably end up in another $100 in misc parts if I don't melt anything in the process.
You can follow along with this build on via this YouTube playlist. Right now I've posted a couple of thought process videos in the planning stages. I've collected most of the parts, and the chassis should be here next week to dive into the metal work.
I have a feeling this one will end up in a different place than my initial planned path, but I do want to stick with a coupled 6SN7 driving a 300B as the end point.
300B DIY Budget Tube Amplifier Build Series - YouTube
Wonderful. A 300B has been in my to-do queue forever, along with a dozen other distractions and inertia. But I have not begun to study a 300B at all other than collecting schemas as I come across them.
There's nothing sacred here, even 300B is just a tube.
The Morrison design is controversial indeed though. I personally dislike it.
The Morrison design is controversial indeed though. I personally dislike it.
Your new chassis looks small for a 300b amp. Why don't you use the original chassis? That was bigger and looked good?
I'm a fan of 2 stage 300b amps myself. Here you have 3 cathode bypass caps which is 2 more than I'd want. Can you eliminate the one on the input stage by using a different operating point with a smaller cathode resistor so it doesn't need to be bypassed? Anyway, I strongly recommend DC Link caps for cathode bypasses, and indeed the last caps before stages in the PSU. In addition to sounding clearer with better tone than everything else I tried, they're smaller and square, which will help your build.
Hope that helps.
I'm a fan of 2 stage 300b amps myself. Here you have 3 cathode bypass caps which is 2 more than I'd want. Can you eliminate the one on the input stage by using a different operating point with a smaller cathode resistor so it doesn't need to be bypassed? Anyway, I strongly recommend DC Link caps for cathode bypasses, and indeed the last caps before stages in the PSU. In addition to sounding clearer with better tone than everything else I tried, they're smaller and square, which will help your build.
Hope that helps.
I think you might be confused, I agree that new one isn't right for this build, and I decided to order a Hammond 17X12X2 chassis. That "original chassis" was a 17X12X3 and I never liked the taller 3 inch size of it. Plus I think the tube placement is wrong for this build as far as reusing it.
This is a work in progress, I'm not fixed on any of the design other than I want to use a 6SN7 to drive a 300B. I'm likely going to use an LED on the cathode of the first, and depending on the drive I need, reconfiguring the second stage one as well. I'm worried about the loss of drive and the change in tone an unbypassed resistor will have but who knows at this point.
I'm about 1/2 way through this build and have settled on the schematic I'm building this to as a starting point. I'll likely go with an LED on the cathode of the first stage as part of the tuning process. I'm building it with a split rail power supply. I have about 6 videos uploaded as I work through the build.
300B DIY Budget Tube Amplifier Build Series - YouTube
300B DIY Budget Tube Amplifier Build Series - YouTube
Attachments
Those LT1085 may need more capacitance on the output, and a diode from output to input.
Check the datasheet, LDO's can become unstable if the ESR of the output capacitor is not low enough.
The spec sheet said to use a tantalum capacitor of that value on the output. Is that wrong? I didn't want to put a huge cap across the heaters. And am curious why a diode would be needed across it. TIA
You do not need protection diodes on that LDO circuit. The load is the filament, not inductive enough, and you do not have huge amounts of capacitance on the output side of the LDO. The ripple should be in the ballpark of 1.5Vp (120Hz, 1.2A, 6.8mF) into the LDO. The filaments will pull the output down when power is removed. Nothing drastic for that LDO that it needs protection.
I’m tempted to accidentally put a 2a3 socket with sufficient power reserves my amp 🙂
I’ve been following your series 🙂
I’ve been following your series 🙂
I'm also tempted to try a 2a3 in place of 300b in one or two of my amps....
Just have a pair of EH, no fancy old US ones.
If its solid tantalum, its likely okay, i gave you the heads up because ive seen LDO's go unstable. The schematic made no mention of the type of capacitor, so i assumed electrolytic.
A big cap has no benefit in heating something that is drawing a constant load, its just that either 22u solid tantalum or 150uF aluminium electrolytics are the reccomended output capacitors to maintain stability. Having to do with the difference in LDO and normal linear regulators.
A diode from output to input is to protect the regulator in case of shorts in front of the regulator, in that case the output capacitor can discharge through the device.
Great series, Keep up the good work.
Indeed, really good and easy to follow videos. I enjoyed some of the other videos on the 6BM8. Terrific!
Yes, it was famous in the 1990s, but does not match many of the amps that have appeared here on diyAudio more recently. I would consider a better driver. Good quality 6SN7s, if they still work, are expensive, and a bad match for a budget design. They don't even perform well, give the large output swing required at the 300B grid.
For best value, a pentode like 6BW7, 6П14П/EL84 or even 6П15П give much better articulated sound.
3-terminal chip regulators in voltage mode can't distinguish the DC voltage it must regulate from the music-signal across the filament (which it needs to leave alone). If you compare them even to the mediocre method of AC-heating, you'll see what I mean.
I agree with you, Rod. I have tried pentode connected 6SJ7, cascoded 6SN7, 6C45P, triode connected 6E5P and pentode connected EL84, with the latter sounding best of all.
I'm still seeking for a potential better driver. Candidates are: triode connected 6J9P, triode connected EL84 and triode connected D3A.
Thanks for explaining, and I'm glad people are enjoying following along with my exploration journey!
While you might not agree with some of my build choices, this part simply isn't true. The Russian 6H8C is a great sounding equal of a 6SN7GT and a NOS pair can be bought from retail stores for less than $20.
NOS 6H8C 6SN7GT Soviet Russia 2 Hole Plate Vacuum Tube - VIVA TUBES
I never claimed this amp is going to be the equal of a $2500-10,000 build with $$$ worth of heater regulation, $$$$$$ worth of top end OT/tubes.
It's a simple, proven, fairly low cost design that I can't imagine won't sound good. Obviously YMMV and you are welcome to make you own video series on your idea of what a Budget 300B build should look like 🙂
Yes, exactly. It's simple, low cost and should sound good. The big advantage is that many commercial amps use this circuit, including Chinese ones. I have a friend that is setting up exactly this circuit and he's finding it very useful.
Here on DIY Audio there are quite a few people who have been searching for the Holy Grail of 300b SE amps. They have gone in a few different directions, not necessarily much more expensive ones though you would tend to factor in a good plate choke or interstage. The preference I've generally seen here is for 2 stage amps. That's what I use myself. It's a stage up in skill from the JE Labs circuit because it can involve pentodes and high gm tubes which can oscillate. So though the JE Labs circuit is something many of us wouldn't build ourselves, there absolutely is a place for a good video of how to build one. And you do good videos!
> The Russian 6H8C is a great sounding equal of a 6SN7GT
The 6H8-C is nominally an equivalent to the 6SN7, but I would not call it an equal. I compared them to the GE and Sylvania when I built the Morrison design in the 1990s, and apart from the difference in sound, the 6H8-C runs the internal conductors of grids and heaters much closer - enough to cause hum in some samples; I am not picking holes in something I haven't tried.
> $$$ worth of heater regulation, $$$$$$ worth of top end OT/tubes
Such an extreme spend is not needed; but a 300B-SE is worth at least, say, $$ for heating and $$$ for an OT. Economising further than this will result in lower performance than simply triode-connecting a KT88, and running it on ac-heat.
The 6H8-C is nominally an equivalent to the 6SN7, but I would not call it an equal. I compared them to the GE and Sylvania when I built the Morrison design in the 1990s, and apart from the difference in sound, the 6H8-C runs the internal conductors of grids and heaters much closer - enough to cause hum in some samples; I am not picking holes in something I haven't tried.
> $$$ worth of heater regulation, $$$$$$ worth of top end OT/tubes
Such an extreme spend is not needed; but a 300B-SE is worth at least, say, $$ for heating and $$$ for an OT. Economising further than this will result in lower performance than simply triode-connecting a KT88, and running it on ac-heat.